Playing Modern is expensive. Wizards of the Coast created the format to be a more cost-friendly (and less broken) format than the Legacy and Vintage circles that became proliferated with old-timers or people with the disposable income to buy a playset of Moxes. Unfortunately with the explosion of popularity the game has had in the last 5 years, many Modern "staples" are skyrocketing in price too. Now Modern has become another expensive format, leaving people who just "want to play with all their cards" frustrated (to say the least).
Wizards really needs to find a way to remedy this quickly, reprinting cards in a meaningful numbers to control costs in the secondary market. Supplemental products like Modern Masters and Eternal Masters are great, but they aren't printed with enough quantity to really mitigate the problem at all. These boutique sets with $10.00 packs have become more of a "Magic Lottery" than a solution to the problem. The reprint problem is compounded by the fact Wizards has discontinued the yearly core set, focusing on the new two block per year paradigm. They claim that they will work to integrate reprints into these new sets, but the Zendikar block failed to reprint the Modern staple Goblin Guide even though it is his home plane; while Shadows of Innistrad had almost no reprints - let alone a meaningful Modern one.
So.... with this being the world we live in what's a player looking to break into the Modern scene to do? Well here's my suggestion of the day - Mono-Red Goblins. The great thing about a mono-colored deck is that the mana base is dirt cheap. You can get by in this deck with 19 mountains. That's value! Goblins (and Elves as an alternative) have a long history in Magic, so there are plenty of cards to choose from. If a card is too expensive you can usually find a cost effective alternative to fill the spot. Here is my decklist that you can use as a starting template - now let's look at some of the superstars of my build!
Goblin Guide - This is a card that should have been reprinted in Battle for Zendikar (see above). Wizards chose to pass on the reprint so now it costs $64.00. Each. That makes a playset $256.00. Good grief. So I'm telling you to play this if you already have it, but for the purpose of having fun in Modern don't go a throw $256.00 at four cards. If you are playing at a Pro Tour Qualifer then you will need it. I almost forgot... the card is amazing - it is a 2/2 for one red mana with haste. Its downside is meaningless given the speed this deck takes out opponents.
Goblin Piledriver - Now for the flip side of the above scenario. In Origins, Wizards made the choice to reprint this card. It hadn't seen the light of day in years (Onslaught, 2002) and a card I never thought they would consider putting back in rotation. Piledriver is a 1/2 creature for two mana with Protection from Blue that also gets +2/+0 when it attacks for every other attacking goblin. The math this card creates on its own is crazy. When combined with haste (granted to all your goblins by Goblin Chieftain) it becomes a surprise that causes many an opponent to scoop up their cards. The reprint of this card made Goblins a competitive deck in Modern and re-energized players' interest in mono-red. Its cost is $7.56 a piece. This is why reprints are so important to the health of the game. Buy four of these if you are brewing a goblin build.
Goblin Grenade - Direct damage. It goes without saying that if you are playing mono-red the first thing you add to a deck is four Lightning Bolts. The O.G. Alpha burn spell is still by far the best. For your second burn spell look no further than this card. In this build it is absurd. For one red mana and sacrificing a goblin you can do five damage to target creature or player. After a huge combat swing you can drop this card in your second main phase, sacrifice a tapped goblin and put your opponent away. The flavor of the card is amazing too - it is a very goblin thing to strap a bomb to your fellow soldier and launch him at the enemy. Great fun - run four.
That's enough to get you started. Do some research, pick some goblins, be creative and sleeve up some fun! Enjoy the deck!